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STOCKHOLDERS' EQUITY (Details 10) (USD $)	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
401(k) Plan
Matching contribution by employer (as a percent)	50.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent	75.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent	100.00%us-gaap_DefinedContributionPlanEmployerMatchingContributionPercent
Period of vesting of matching contributions		4 years	4 years
Service period of employees		4 years	4 years
Unvested amount under the defined-contribution savings plan to be amortized	$ 273,000gern_DefinedContributionPlanUnvestedEmployerMatchingContributionsAmountToBeAmortized
Geron 401K Plan
401(k) Plan
Operating expenses as a result of accelerated vesting of unvested prior employer matches for employees affected by recent restructurings		266,000gern_DefinedContributionPlanAcceleratedVestingAmountInUnvestedPriorEmployerMatchesForEmployeesAffectedByRestructuring / us-gaap_PlanNameAxis = gern_Geron401KPlanMember	370,000gern_DefinedContributionPlanAcceleratedVestingAmountInUnvestedPriorEmployerMatchesForEmployeesAffectedByRestructuring / us-gaap_PlanNameAxis = gern_Geron401KPlanMember
Research and development | Geron 401K Plan
401(k) Plan
Operating expense recorded for vested portion of employer matching contribution	175,000us-gaap_DefinedContributionPlanCostRecognized / us-gaap_IncomeStatementLocationAxis = us-gaap_ResearchAndDevelopmentExpenseMember / us-gaap_PlanNameAxis = gern_Geron401KPlanMember	156,000us-gaap_DefinedContributionPlanCostRecognized / us-gaap_IncomeStatementLocationAxis = us-gaap_ResearchAndDevelopmentExpenseMember / us-gaap_PlanNameAxis = gern_Geron401KPlanMember	616,000us-gaap_DefinedContributionPlanCostRecognized / us-gaap_IncomeStatementLocationAxis = us-gaap_ResearchAndDevelopmentExpenseMember / us-gaap_PlanNameAxis = gern_Geron401KPlanMember
General and administrative | Geron 401K Plan
401(k) Plan
Operating expense recorded for vested portion of employer matching contribution	$ 143,000us-gaap_DefinedContributionPlanCostRecognized / us-gaap_IncomeStatementLocationAxis = us-gaap_GeneralAndAdministrativeExpenseMember / us-gaap_PlanNameAxis = gern_Geron401KPlanMember	157,000us-gaap_DefinedContributionPlanCostRecognized / us-gaap_IncomeStatementLocationAxis = us-gaap_GeneralAndAdministrativeExpenseMember / us-gaap_PlanNameAxis = gern_Geron401KPlanMember	259,000us-gaap_DefinedContributionPlanCostRecognized / us-gaap_IncomeStatementLocationAxis = us-gaap_GeneralAndAdministrativeExpenseMember / us-gaap_PlanNameAxis = gern_Geron401KPlanMember

X

- Definition

Represents the accelerated vesting amount in unvested prior employer matches for employees affected by restructuring under the defined contribution plan.

+ References

No definition available.

+ Details

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X

- Definition

Represents the period of service of an employee must complete to be fully vested in an employer matching contribution.

+ References

No definition available.

+ Details

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X

- Definition

Represents the unvested amount of employer matching contributions to be amortized under the defined-contribution savings plan as the corresponding years of service are completed by the employees.

+ References

No definition available.

+ Details

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X

- Definition

Represents the period over which matching contributions invested in common stock will vest ratably for each year of service completed by the employee.

+ References

No definition available.

+ Details

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Namespace Prefix:	gern_
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- Details

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- Definition

The amount of the cost recognized during the period for defined contribution plans.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 715

-SubTopic 70

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6414718&loc=d3e28014-114942

+ Details

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- Definition

Percentage of employees' gross pay for which the employer contributes a matching contribution to a defined contribution plan.

+ References

No definition available.

+ Details

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- Details

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- Details

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- Details

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